query taking long time for execution inthis case what should i do Expand / Collapse

hi all
1. application team complaining that queries are executing slow and want reason for that in this case what should i do [or ] follow

2. customer complaining that the data which they are uploading is taking so much of time and continues to the next time with out completion
(1.) inthis case when they have to upload the next day data customer canelling the previous day data which is still uploding staus
inthis case what should i follow

3..mayin this by cheking sp_who2 active [or ] or using dmvs [or ] dealocks what ever the things which may kill the process in activity monitor

customer dose not want to us to kill any process with out approvalll....

so please guide or suggest us with root cause by that i can handel this issue woludn't repeat again